1. Field of the Invention
The invention is directed to a process for printing a halftone image according to image data, wherein every image datum describes the halftone step of a halftone dot of an artwork or model image. The process utilizes processing frames, having positions for the image data of a plurality of adjacent halftone dots. The image data are adopted in the processing frame in the arrangement of the halftone dots described therein. Color shade or step values are obtained from the image data contained in the processing frame and associated with print positions of a printing raster or screen, and color dots with graduated color gradations are optionally printed at the print positions produce the halftone steps. The color gradation of every color dot is adjusted according to the color step value associated with the respective print position.
Within the scope of the present invention, printing is fundamentally understood as the arrangement of color dots on a printed medium.
A process for printing a halftone image with a plurality of halftone steps is known from DE-PS 35 25 011, in which each image datum describes the halftone step of a halftone dot of a model image. A matrix including a number of print positions of a printing screen is associated with every image datum by observing threshold values. The print positions of every matrix can be covered by color dots of graduated color gradations (various sizes). In order to achieve a sufficient number of halftone steps in this known process, the matrix must include a corresponding number of print positions. However, the local resolution of the reproduced halftone image decreases considerably in comparison to the model image as the number of print positions per matrix increases. Therefore, in order to achieve a good image reproduction in spite of this, print heads with a very high resolution must be used in the known process. This technique raises the price of a corresponding printing device.
2. Description of the Related Art
The invention is based on a process for printing a halftone image known from EP 0 240 202 A1. In this process, also, there is a model image in the form of image data, wherein every image datum is associated with a halftone dot of the model image and describes its halftone step. Processing frames are used in this known process and every processing frame has positions for receiving a plurality of image data of adjacent halftone dots of the model image. The image data are adopted into the positions of the processing frames in the arrangement of halftone dots described by them. Color step values are obtained from the image data contained in the processing frames by averaging and these color step values are assigned to or associated with print positions of a printing screen. A color dot with graduated color gradations can be outputted optionally at each print position of the printing screen to produce the halftone steps. The color gradation of every color dot is determined by a color step value associated with one of the respective print positions. In the known process, the image data of a processing frame are evaluated jointly by averaging or interpolation and in this way a color step value is achieved which is associated with a number of print positions in the printing screen corresponding to the number of image data per processing frame. In this way, in contrast to the process known from DE-PS 35 25 011, a large number of halftone steps can be produced when using a print head whose resolution approximately corresponds to the resolution of the model image. This can be attributed to the number of print positions with which the obtained color step value is associated corresponds at least approximately to the number of image data contained in the processing frame. However, in the process known from EP 0 240 202 there is a considerably reduced local resolution in the region of sharp contrast jumps in the model image. That is, a contrast jump represented by image data within a processing frame cannot be shown in sharp definition due to the averaging. The human eye is extremely sensitive to such blurriness (mixing effects). This effect is aggravated as the number of image data or positions per processing frame increases.